Relax and Sleep has a variety of sounds, none of which are extremely high in quality, but this sound app brings something I think is a little bit original. To paraphrase AppBrain, I'm able to play a mix of sounds and adjust volumes to make my own ideal ambient melodies. I like to get creative and combine the piano with the train. If that doesn't warp my mind, nothing will.
Lightning Bug is similar to Relax and Sleep in that I can make combinations of sounds, but this is a little more themed. I've always enjoyed that the sounds don't give the impression that they're in a loop. The randomness actually helps keep me from creating a pattern which defeats the purpose of these apps.
Lastly, White Noise, which is a paid app provides a very large library of sounds and I think the most seamless settings for sleep, timers, etc. This is the app that I have and use on my iPod Touch, I do not have this for Android, but out of all the apps, it's helped me the most get to sleep or settle my thoughts at bedtime.
Of course for me, the best sound of all is the sound of silence, but when a little noise helps, these apps always do the trick.
